1. Field of the Invention
The present invention relates to active carbon suitable for use in polarizable electrodes for an electric double layer capacitor making use of an electrolytic organic solvent solution.
2. Description of the Background Art
Since an electric double layer capacitor has a large capacity of the farad level and is also excellent in charge and discharge cycle properties, it is used in applications such as back up power supplies for electronic equipment and batteries for automobiles.
In this electric double layer capacitor, for example, as illustrated in FIG. 4, a pair of polarizable electrodes 1, 1 composed of active carbon are arranged in an opposed relation to each other through a separator 2, and the polarizable electrodes 1, 1 are impregnated with an organic solvent solution of a tetraalkylammonium salt or the like as an electrolytic solution so as to function as positive and negative electrodes, respectively. In the electric double layer capacitor illustrated in FIG. 4, the polarizable electrodes 1, 1 opposed to each other through the separator 2 are contained in an aluminum container 3 which is closed with an aluminum lid 5 through a packing 4. In the above-described construction, the container 3 and the lid 5 come into separate contact with the polarizable electrodes 1, 1 in such a manner that the container 3 serves as a current collector member on the cathode side to the polarizable electrode 1, and the lid 5 serves as a current collector member on the anode side to the other polarizable electrode 1.
Although active carbon having fine pores is used for the polarizable electrodes for such an electric double layer capacitor, there is a demand for development of active carbon capable of more heightening the capacity density of the polarizable electrodes for the purpose of providing a smaller and lighter electric double layer capacitor having a greater capacity.
Therefore, various properties of active carbon have been investigated with a view toward providing active carbon capable of heightening the capacity density of the polarizable electrodes. For example, the hypothesis that "A capacity density per weight of active carbon in an electrode has an almost linear proportional relationship with a specific surface area of the active carbon, and the capacity of an electric double layer on the active carbon electrode is about constant without being affected by the kind of carbon and pore characteristics thereof" has been proposed (Electrochemistry, 59, No. 7, pp. 607-613, 1991).
For example, Japanese Patent Application Laid-Open No. 302735/1995 describes active carbon the capacity density per weight of which has been heightened by subjecting a carbonaceous material to activation to enlarge its specific surface area on the basis of the above hypothesis. The active carbon described in the publication is such that the carbonaceous material is subjected to the activation, thereby forming pores suitable for adsorption of the electrolytic solution ions.
However, the active carbon described in the publication involves a disadvantage that its capacity density per volume is lowered when the activation is conducted until the capacity is developed.
In order to solve the above disadvantage, the present inventors attempted to investigate the above hypothesis. The investigation by the present inventors revealed that when a theoretical value of the capacity of an electric double layer capacitor comprising polarizable electrodes making use of such active carbon is found from a value observed by a mercury electrode or the like as the capacity of the electric double layer of the active carbon and a specific surface area of the active carbon, the theoretical value may not consist with the found value of the capacity of the electric double layer capacitor in some cases.
For example, assuming that the capacity of the electric double layer capacitor so constructed that a pair of polarizable electrodes 1, 1 are arranged with a separator 2 held therebetween as illustrated in FIG. 4 is C.sub.0, and capacities of the polarizable electrodes 1, 1 are C.sub.1 and C.sub.2, the following equation is satisfied: EQU 1/C.sub.0 =1/C.sub.1 +1/C.sub.2 ( 1)
Here, the capacity of the electric double layer of the active carbon observed by the mercury electrode is about 20 .mu.F/cm.sup.2. Therefore, the capacity of the electric double layer of active carbon having a specific surface area of 1,500 m.sup.2 /cc amounts to: EQU 20 (.mu.F/cm.sup.2).times.1,500 (m.sup.2 /cc)=300 (F/cc)
Then, 300 (F/cc) is substituted for C.sub.1 and C.sub.2 in the equation (1) to find C.sub.0. As a result, C.sub.0 amounts to 150 (F/cc). Since C.sub.0 is a capacity for two volumes of the polarizable electrode 1, the theoretical capacity of the electric double layer capacitor should amount to 75 F/cc obtained by dividing the C.sub.0 value by 2.
In reality, the capacity of the electric double layer capacitor amounts to only about 13 F/cc. Even when different kinds of active carbon having the same specific surface area are used, the capacities of the resultant electric double layer capacitors may be entirely different from each other in some cases.
Then, the present inventors paid attention to the fact that the above hypothesis is based on the specific surface area measured by nitrogen gas absorption in accordance with the BET method, and attempted a special image analysis capable of analyzing even fine pores smaller than the limit by the BET method by using an image through a transmission electron microscope, thereby investigating the relationship between capacity density and specific surface area as to various kinds of active carbon for electrodes different in specific surface area. As a result, the conclusion that there is no linear proportional relationship between them, and so factors affecting the capacity exist in some others was reached.
The present inventors carried out a further investigation on the basis of this finding. As a result, it was found that in the above electrolytic solution, the organic solvent is solvated with ions of an electrolyte such as the tetraalkylammonium salt or the like to form electrolytic solution ions, and so many pores having a pore size suitable for adsorption of the electrolytic solution ions are formed in active carbon, thereby providing active carbon excellent in capacity density per volume.
Based on this finding, the reason why in the active carbon described in Japanese Patent Application Laid-Open No. 302735/1995, the capacity density per volume is lowered when the activation is performed until the capacity is developed is considered. The active carbon is such that the carbonaceous material is subjected to the activation, thereby forming pores suitable for adsorption of the electrolytic solution ions. However, it is considered that the pore size distribution of pores formed by such activation widely ranges from a small size to a large size. As a result, it is considered that the number of pores having a pore size suitable for the adsorption of the electrolytic solution ions relatively decreases in such active carbon, in other words, the number of pores noncontributory to capacity increases, so that a sufficient capacity density per volume cannot be obtained.
Further, the present inventors found active carbon suitable for use in electrodes for an electric double layer capacitor, in which the pore size suitable for the adsorption of the electrolytic solution ions takes a mode in its pore size distribution, on the basis of the above finding. Such active carbon was previously applied for patent (Japanese Patent Application No. 46912/1996). The active carbon described in the above specification is such that a carbonized product obtained by calcining a vinyl chloride resin is activated with an alkali at a temperature ranging from 400.degree. to 1,000.degree. C. The mode in its pore size distribution is within a range of 10-20 angstroms, which are pore sizes suitable for adsorption of the electrolytic solution ions. According to such active carbon, a polarizable electrode high in electrode density and capacity density per volume can be formed because the mode in its pore size distribution falls within the above range. In particular, when the alkali activation is performed for 15-20 hours at the temperature within the above range, the capacity density per volume becomes a maximum value.
Incidentally, the mode in the above specification means a value of a pore size which shows the highest relative frequency in the frequency distribution of pore sizes, which is found from a power spectrum obtained by converting the image of the active carbon through a transmission electron microscope into a binary image and subjecting this binary image to Fourier transformation.
According to such active carbon, a polarizable electrode high in capacity density per volume can be constructed, and an electric double layer capacitor high in energy density can hence be constructed by such polarizable electrodes. However, there is a demand for development of active carbon more improved.